Semiconductor strain sensor with controlled sensitivity

ABSTRACT

A field effect transistor is used as a controlled sensitivity strain sensor by varying the gate-to-source voltage or other equivalent gate electrode parameter to change the strain sensitivity of conduction through the channel. In general, the gage factor decreases as the gate-to-source voltage is increased. Applications include temperature compensation and scale linearization.

United States Patent 3,582,690 [72] Inventor Alexander J. Yerman3,351,786 11/1967 Muller et al. 307/308X Scotia, N.Y. 3,492,861 2/1970Jund 307/308 [211 App]. No. 831,472 3,510,696 5/1970 Bargen et al307/308 [22] Filed June 9, 1969 Patented June 1971 Primary ExaminerDonald D. Forrer [73] Assignee General Electric Company [54]SEMICONDUCTOR STRAIN SENSOR WITH Assistant Examiner-B. P. DavisAttorneys-John F Ahem, Paul A. Frank, Donald R.

Campbell, Joseph B. Forman, Frank L. Neuhauser and Oscar B. WaddellCONTROLLED SENSITIVITY 6 Claims, 7 Drawing Figs. [52] US. Cl 307/308,73/885, 307/278 [SI] ll'll. CI "03k 17/00 ABSTRACT; A field effecttransistor is used as a controlled [50] Field of Search 307/278,sensitivity strain sensor by varying the gate-to-source voltage 308 orother equivalent gate electrode parameter to change the strainsensitivity of conduction through the channel. In [56] References Cnedgeneral, the gage factor decreases as the gate-to-source volt- UNITEDSTATES PATENTS age is increased. Applications include temperaturecompensa- 3,248,654 4/1966 Shiraqaki 307/308X tion and scalelinearization.

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PATENTED JUN 1 I971 SHEET 1 UF 2 P- CHAIV/VIZ H/ls At orneySEMICONDUCTOR STRAIN SENSOR WITH CONTROLLED SENSITIVITY This inventionrelates to semiconductor strain gages which exhibit a change inresistance as a function of an applied strain, and more particularly tosemiconductor strain sensors with a controlled strain sensitivity.

Certain semiconductors such as silicon and germanium have large gagefactors and consequently have been employed as stain gages for sensingvarious types of stresses. The high gage factors, where gage factor isdefined as the fractional change in resistance per unit strain, are dueto the fact that the piezoresistive effect resulting from thestrain-induced modulation of the conduction mechanism of thesemiconductor is especially pronounced in these materials. A desirablefeature in the use of semiconductor strain sensors would be the abilityto control the strain sensitivity of the sensor. This refers to thecapability of changing in a predetermined manner the strain sensitivityor gage factor of the sensor. There are many applications where suchcontrol would be useful, as for instance, to compensate for the normaldecrease in gage factor of semiconductor strain gages as the temperatureincreases. Other possible uses include compensation to improve frequency response, improving selectivity for a particular signal input, andscale linearization, expansion, or compression.

The field-effect transistor is suitable to be employed as a sensor formeasuring strain. Basically, the field effect transistor comprises alayer of semiconductor material constituting a channel for majoritycharge carrier flow between source and drain electrodes, theconductivity of the channel being controlled by the voltage applied to agate electrode overlying the channel between the source and drainelectrodes. When subjected to a strain, the resistance of the conductingchannel changes in dependence on the amount of the strain. It has notheretofore been recognized, however, that the gage factor, or strainsensitivity, of a field effect transistor, or a device by whatever namehaving a similar physical structure, can be controlled by means ofvarying the voltage on the gate electrode.

Accordingly, an object of the invention is to provide a new and improvedsemiconductor strain sensor having a controllable or variable strainsensitivity.

Another object is the provision of a semiconductor strain sensor with avoltage-controlled gage factor, taking the form of a field-effecttransistor employed in a circuit as a device for sensing strain.

Yet another object is to provide a compensated strain gage deviceincorporating a semiconductor strain sensing field-effect transistorwhose strain sensitivity is varied as a function of a preselectedparameter such as temperature or frequency, or input strain magnitude.

In accordance with the invention, a controlled sensitivity semiconductorstrain sensor comprises a strain sensitive device having first andsecond electrodes and a body of semiconductor therebetween providing achannel for charge carrier flow, and a third electrode overlying thechannel to which a voltage is applied for controlling the conductivityof the channel. Means are provided for applying a voltage between thefirst and second electrodes and between the first and third electrodes,and for subjecting the device to a strain. A selected third electrodeparameter is varied to thereby change the strain sensitivity of thedevice, which is preferably a field effect transistor wherein thegate-to-source voltage is the parameter that is adjustable and modulatesthe strain sensitivity of the channel.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of several preferred embodiments of the invention, asillustrated in the accompanying drawings wherein:

FIG. l is a cross-sectional isometric view of one type of insulated-gatefield effect transistor that can be used as a voltage controlled strainsensor according to the teaching of the invention,

FIG. 2 is a typical set of current-voltage characteristic curves fordifferent values of gate-to-source voltage for the field-effecttransistor shown in FIG. 1;

FIG. 3 is a perspective view ofa cantilever beam device having anintegrally formed field effect transistor strain sensor;

FIGS. 4 and 5 each show a series of curves of gage factor vs.gate-to-source voltage for different ambient temperatures for each ofthe two values of bridge voltage, respectively;

FIG. 6 is a schematic circuit diagram ofa bridge circuit with a fieldeffect transistor strain sensor connected so that the gate voltage isvaried as a function of temperature, to compensate for changes insensitivity due to temperature; and

FIG. 7 is similar to FIG. 6 but illustrates a field effect transistorstrain sensor whose sensitivity is controlled for scale linearizationpurposes.

Although other types of field effect transistors can be used assensitivity controlled strain sensors, the device 11 shown in FIG. 1 byway of illustration is a p-channel enhancement modemetal-oxide-semiconductor (MOS) field effect transistor. This form ofinsulated gate field effect transistor is well known in the art and willbe described only briefly. The device 11 is formed at one surface of ann-type substrate 12 made of silicon or other suitable semiconductormaterial and comprises two spaced regions 13 and 14 of heavily dopedp-type silicon providing respectively the source and drain electrodes.Ohmic metallic contacts 15 and 16 made for instance of aluminumpartially overlie the source electrode 13 and drain electrode 14 andmake connection to a supply of source-to-drain voltage (not here shown).A metallic gate electrode 17 is deposited on a layer of insulatingmaterial of silicon dioxide, for example, which in turn overlies thesurface of the substrate 12 between the source electrode 13 and drainelectrode 14 and partially overlaps each of these electrodes. A separatecontact is usually provided for the substrate which is not shown. Thisis frequently connected to the source electrode and such a connection isassumed in the following description. The gate electrode 17 is connectedto a suitable supply of gate-to-source voltage. Assuming that the drainelectrode 14 is biased negative with respect to the source electrode 13,then a negative voltage applied to the gate electrode 17 in excess of acertain threshold voltage characteristic of the device creates anelectrostatic field that attracts the positively charged chargecarriers, or holes, in the n-type substrate 12 and creates a p-channel19 between the source and drain electrodes 13 and 14 thereby renderingthe device conductive. As will be observed from a typical set ofcurrent-voltage characteristic curves given in FIG. 2, the outputcurrent I for a preselected value of drain-to-source voltage V increasesas the gate-to-source voltage V increases.

As has been pointed out, the filed effect transistor can be used as asemiconductor strain sensor. For such applications, the field effecttransistor 11 is suitably mounted to be subjected to a strain in orderto sense force, pressure, strain, acceleration, displacement, or anyother parameter of interest by suitable coupling of the parameter togenerate the strain required. The field effect transistor strain sensormay be for example mounted on or integrally formed in or on a flexiblediaphragm to sense pressure, or a cantilever beam to sense force,displacement, or acceleration. In the arrangement shown in FIG. 3, thefield effect transistor strain sensor 11 is formed integrally on or inone surface of a semiconductor cantilever beam 21 secured at one end toa support 22, to sense the strain induced by a force applied to the freeend of the beam. When the field effect transistor 11 is connected in asuitable resistance measuring circuit such as a resistance divider or aWheatstone bridge, the resistance of the strain sensor varies inaccordance with the strain to which it is subjected, which in turn is afunction of the applied force. Preferably, the beam 21 is made of singlecrystal silicon, and the gage factor of the field effect transistorstrain sensor 11 is determined in known manner to a first order by thecrystallographic plane and the direction of the conduction channelwithin the plane. To review the latter briefly, the piezoresistancecoefficients in a crystal or semiconductor material such as silicon arerelated to the crystallographic axes of the crystal In silicon or othermaterials having diamond-cubic symmetry. three coefficients serve tocompletely define the piezoresistance characteristics. These fundamentalpiezoresistance coefficients 11' 11, and 1r serve to define thesensitivity to longitudinal, transverse, and shear stresses respectivelyOf more practical usefulness are the piezoresistance constants which arederived from these coefficients and includes the dependence on directionin the crystal. From these, it is customary to derive a set of gagefactors expressed in terms of longitudinal and transverse strains. Thesegage factors also incorporate the anisotropic elastic coefficients ofthe crystal. In general, for a given material, they vary with directionin the crystal and with dopant concentration. Specifically, thelongitudinal gage factor for a specific direction within the crystalrefers to the fractional change in resistance per unit strain whencurrent and strain directions are parallel:

TABLE 1 Effect of Crystal Direction on Longitudinal and Transverse GageFactor in p-Type Silicon (7.8ncm.)

Direction Current Strain GFr. GFT

In general, there is found the same first order dependence on crystaldirection in field effect transistors taking the channel currentdirection as the pertinent parameter. For example, in the case of thecantilever shown in FIG. 3, when the plane of the cantilever is (110),and the direction of current flow through the channel is parallel to thelong dimension of the cantilever is a lll direction, maximum sensitivityto bending strain results for a p-channel field effect transistor. Withreference to FIGS. 1 and 3, the device 11 will then be oriented with thedirection of charge carrier flow in the pchannel 19 parallel to thelongitudinal axis of the beam 21. This is, of course, not the onlyconfiguration ofpractical value but is illustratory of the fact thatreasonably high gage factors in the order of 100 or more can be obtainedby properly choosing the plane ofsilicon from which the beam 21 is made,and the orientation of the field effect transistor strain sensor 11 onthe beam. The criteria for making these selections are exactly the sameas for other types of semiconductor strain sensors.

The field effect transistor when used as a strain sensor is differentfrom conventional strain sensors, however, in that the strainsensitivity of conduction through the channel is affected by the voltagepresent on the gate electrode. FIGS. 4 and each show typical variationsin gage factor as a function ofthe voltage applied between the gate andsource electrodes (V at three different temperatures as indicated,namely 25, 50, and 75 C. This data was obtained from a p-channelenhancement mode metal-oxide silicon field effect transistor formed on a(111) plane of single crystal silicon with the channel strained in thelongitudinal direction, i.e., l10 The test circuit was a Wheatstonebridge circuit having in the other arms a reference field effecttransistor and two identical resistors, and FIG. 4 shows the dataobtained when the bridge supply voltage, V,,, is 1 volt, while in FIG. 5V is 9 volts.

Neglecting for a moment the extreme left-hand portions of the curves,FIGS. 4 and 5 illustrate the large decrease in gage factor as V isincreased. The change in gage factor for the higher drain-to-sourcevoltages used to obtain the data in FIG. 5 can be a ratio of almost 2:1.The erratic behavior below 4 volts in FIG. 4 and below about 6 volts inFIG. 5 is probably due to another effect becoming significant. It willbe noted that the gage factors obtained with a bridge voltage of 9 voltsare slightly higher than the corresponding values obtained using abridge voltage of 1 volt, and the temperature sensitivity of the gagefactor is somewhat higher also. This gage factor temperature sensitivitybehavior compares qualitatively with that of p-type silicon ofmoderately high dopant concentration. Although the reasons for thedecrease in gage factor as the gate voltage is increased are not knownwith certainty, a possible explanation accounting for this variation ingage factor with gate voltage is the change in carrier concentration andmobility with gate voltage and channel voltage, respectively. At lowvalues of V the number of charge carriers in the channel 19 (see FIG. 1)by reason of the electrostatic field created by the gate electrode 17 islow, and since the carrier concentration is low, there is a resultinghigh gage factor. As carrier concentration is increased due to increasesin V or the mobility of the charge carriers is increased due to thereduction of V the gage factor is reduced. This is somewhat analogous tothe changes in gage factor as a function of doping level in silicon.Such a mechanism also would account for the variation in temperaturesensitivity observed. A higher temperature sensitivity is expectedcoincident with higher gage factors, and hence lower carrierconcentration. That the relationship between V and gage factor is notdue to power dissipation and self-heating of the channel (which might beexpected to increase channel temperature as V is increased), can beruled out by the fact that the gage factors are higher for the casewhere V =-9 volts and channel power dissipation is almost times higherthan the corresponding cases where V =-volt.

Other types of insulated gate field effect and also junction fieldeffect transistors can be employed in a similar manner as sensitivitycontrolled semiconductor strain sensors. More specifically, there arefour types of metal-oxide-semiconductor field effect transistorsincluding in addition to the pchannel enhancement mode transistor thathas been mentioned, an n-channel enhancement mode transistor and bothp-channel and n-channel depletion mode transistors. The n-channelenhancement mode type is similar to the p-channel enhancement modetransistor but comprises heavily doped n-type region source and drainelectrodes on a p-type substrate, and the charge carriers that the formthe conducting channels are electrons instead of holes. Depletion-typetransistors have a physical structure similar to that shown in FIG. 1but are fabricated with source, drain, and channel regions made of thesame conductivity type material to yield substantial drain current atzero gate bias. As appropriate gate polarity voltage is applied to thegate electrode, increasing the gate voltage causes the electrostaticfield to deplete the charge carriers in the channel until the channel ispinched off and the device is nonconductive. Another type of insulatinggate field effect transistor is known as the thin film transistor andcommonly comprises an evaporated thin film of semiconductor such ascadmium sulfide with a control gate insulated from the semiconductor.The thin film transistor is described more fully in the Proceedings ofthe IRE, June 1962, pages 1462-1469. The junction field effecttransistor essentially comprises a high resistivity semiconductorconstituting a channel for majority carrier flow between 2 ohmic sourceand drain contacts, with one or two regions of opposite conductivitytype and high doping providing the gate electrode or electrodes. Withthe gate reverse biased relative to the channel using a low biasvoltage, a large current can flow through the channel from source todrain, however when the reverse bias is increased a point is reachedwhere the entire channel is pinched off and no current can flow.

The foregoing review of different types of field effect transistors isnot necessarily comprehensive and the invention is not limited to thosewhich have been mentioned. In fact, any semiconductor device by whatevername which has the essential structural and operating characteristics ofa field effect transistor can be used as a sensitivity controlled strainsensor. Basically, the device comprises a layer of semiconductormaterial constituting a channel for majority charge carrier flow betweensource and drain electrodes, where the conductivity of the channel iscontrolled by the voltage applied to a gate electrode overlying at leasta portion of the channel between the source and drain electrodes. All ofsuch devices when used as a strain sensor are characterized by theability to control the strain sensitivity by reason of the fact that thegage factor of the device changes as the gate voltage is increased.

Two applications for the controlled variable sensitivity semiconductorstrain sensor will be discussed, although other ways of utilizing theeffect are possible and apparent to those conversant with instrumenttransducer design. These are to compensate for changes in sensitivity ofa semiconductor strain sensor due to temperature changes, and for scalelinearization purposes to compensate for the inherent nonlinearity ofsemiconductor strain sensors. In FIG. 6, a full bridge circuit is shownthat includes a temperature compensated field effect transistor strainsensor. The circuit comprises two resistors 24 and 25 and two MOS fieldeffect transistors 26 and 27 connected as the four arms of a Wheatstonebridge that is energized by a source of potential E, connected betweenthe two DC supply terminals 28 and 29. The unbalance output voltage Eappears between the other two junctions of the bridge. Transistor 26 isthe strain sensor while transistor 27 is an identical type unstrained orreference transistor. A variable source-togate voltage that changes as afunction of the temperature is applied in parallel to the gates of bothtransistors. To this end, a resistor voltage divider network comprisingthe series connected resistor 30, temperature sensitive thermistor 31and resistor 32 is connected between the DC supply terminals, and thesource and gate electrodes of each transistor 26 and 27 are coupledacross the last two mentioned resistive elements. The resistance valuescan be chosen such that as the resistance of the thermistor 31 changesin accordance with the second value of temperature, the variablegate-to-source voltage that is developed changes the gage factor of thestrain sensor transistor 26 to compensate for the variation insensitivity due to temperature. The gate voltage applied to unstrainedtransistor 27 is the same as that applied to strained transistor 26 inorder to maintain constant bridge relationships.

In FIG. 7 the controlled sensitivity field effect transistor 26 isemployed in a similar Wheatstone bridge arrangement that includescircuitry for compensating for nonlinearity of readout. The typicalp-type silicon AR/R vs. strain characteristic, for example, is notlinear. in this circuit the gate-tosource voltage is varied as afunction of the unbalance output voltage E,, of the bridge. For thispurpose, a resistor voltage divider comprising resistors 34, 35, and 36are connected between the DC supply terminals 28 and 29, where terminal28 is assumed to be negative while terminal 29 is grounded, and thejunction point of resistors 34 and 35 is coupled to the gates oftransistors 26 and 27, the source electrodes of which are grounded. Theunbalance voltage E, appears between ground and the output of adifferential amplifier 37 whose inputs are respectively the voltages atthe junctions between resistor 24 and transistor 26, and resistor 25 andtransistor 27, in

the bridge. The output of differential amplifier 37, which must benegative going relative to ground, is also fed back through resistor 38to the junction between resistors 35 and 36, and a portion of it iseffective in the resistor divider network comprising resistors 34, 35,and 36 to change the voltage at the junction of resistors 34 and 35,which is the gate voltage of transistors 26 and 27, in a direction tocompensate for the inherent scale nonlinearity. For this circuit, as thestrain is increased the output voltage E increases and goes morenegative, and the feedback effect drives the gate-to-source voltage morenegative than it would be otherwise. This is in the direction FIGS. 4and 5) to decrease the gage factor and consequently linearize the outputvoltage.

While not here illustrated, enhanced strain sensitivity can be achievedby cascading similar type field effect transistors that are subjected tothe same stress, e.g., by applying them to opposite sides of acantilever beam, or by employing two complementary type MOS transistorsso that one has a positive output and the other has a negative-outputand the outputs are added algebraically.

In summary, it has been found that a controlled variable sensitivitysemiconductor strain sensor can be provided by a field effect transistorby varying the gate-to-source voltage to thereby modulate the gagefactor. The effect is applicable in any of the various types of fieldeffect transistors, or in semiconductor structures basically like thatof the field effect transistor, and is not necessarily restricted tochanging the gate voltage, since the same result could be accomplishedby changing some other gate electrode parameter such as by physicallymoving the gate electrode (the gate voltage remaining constant) tomodulate the conduction mechanism of the semiconductor material in anequivalent manner. The con trolled sensitivity semiconductor strainsensor can be implemented by simple additions to conventionalstrain-measuring circuitry and has utility in various instrumenttransducer applications where some electromechanical parameter is to bemeasured, In particular it provides a new and improved means forimproving the characteristics of such instrument transducers byproviding a mechanism for temperature compensating or linearizing thescale of these transducers.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

What i claim as new and desire to secure by Letters Patent of the Unitedis:

l. A controlled sensitivity semiconductor strain sensor comprising:

a strain sensitive device having first and second electrodes and a bodyof semiconductor therebetween providing a channel for charge carrierflow, and a third electrode overlying at least a portion of the channelto which a voltage is applied for controlling the conductivity ofthechannel,

means for applying a voltage between said first and second electrodesand between said first and third electrodes,

means for subjecting said device to a strain, and

means for varying a selected third electrode parameter to thereby changethe strain sensitivity of the device comprising means for sensing aphenomenon to be controlled, and means for varying the selected thirdelectrode parameter in accordance with the sensed value of thephenomenon to be controlled.

2. A controlled sensitivity semiconductor strain sensor devicecomprising:

a field effect transistor having source and drain electrodes and a bodyof semiconductor therebetween providing a channel for majority chargecarrier flow, and a gate electrode overlying at least a portion of thechannel to which a voltage is applied for controlling the conductivityof the channel,

means for subjecting said field effect transistor to a strain,

circuit means for applying voltage between said source and drainelectrodes and for detecting the change in resistance that occurs as theresult of the strain, and

means for applying an adjustable gate-to-source voltage that is afunction of a phenomenon to be controlled, to thereby change the strainsensitivity of the field effect transistor.

3. A device as set forth in claim 2 wherein said circuit means is abridge circuit including at least two field effect transistors, and

said means for applying an adjustable gate-to-source voltage applies thesame gateto-source voltage to both of said field effect transistors.

4. A device as set forth in claim 3 wherein said field effecttransistors are p-type silicon insulated gate field effect transistors.

5. A device as set forth in claim 2 wherein said means for applying anadjustable gate'to-source voltage that is a function ofa phenomenon tobe controlled comprises means for sensing the ambient temperature, andmeans for generating a voltage that is a function of the sensed value oftemperature.

6. A device as set forth in claim 2 wherein said circuit means includesmeans for generating an output voltage indicative of the change inresistance, and

said means for applying an adjustable gate-to-source voltage that is afunction of a phenomenon to be controlled includes feedback means forcoupling at least a portion of said output voltage thereto to result inlinearizing said output voltage.

2. A controlled sensitivity semiconductor strain sensor devicecomprising: a field effect transistor having source and drain electrodesand a body of semiconductor therebetween providing a channel formajority charge carrier flow, and a gate electrode overlying at least aportion of the channel to which a voltage is applied for controlling theconductivity of the channel, means for subjecting said field effecttransistor to a strain, circuit means for applying voltage between saidsource and drain electrodes and for detecting the change in resistancethat occurs as the result of the strain, and means for applying anadjustable gate-to-source voltage that is a function of a phenomenon tobe controlled, to thereby change the strain sensitivity of the fieldeffect transistor.
 3. A device as set forth in claim 2 wherein saidcircuit means is a bridge circuit including at least two field effecttransistors, and said means for applying an adjustable gate-to-sourcevoltage applies the same gate-to-source voltage to both of said fieldeffect transistors.
 4. A device as set forth in claim 3 wherein saidfield effect transistors are p-type silicon insulated gate field effecttransistors.
 5. A device as set forth in claim 2 wherein said means forapplying an adjustable gate-to-source voltage that is a function of aphenomenon to be controlled comprises means for sensing the ambienttemperature, and means for generating a voltage that is a function ofthe sensed value of temperature.
 6. A device as set forth in claim 2wherein said circuit means includes means for generating an outputvoltage indicative of the change in resistance, and said means forapplying an adjustable gate-to-source voltage that is a function of aphenomenon to be controlled includes feedback means for coupling atleast a portion of said output voltage thereto to result in linearizingsaid output voltage.